YC is an accelerator, and as such, needs to accelerate /something/. If you join too early, then it's almost a distraction to getting the product out and talking to users. However, hit the sweet spot and YC is an invaluable resource to help you grow.
On the other hand, YC opens the door to so many opportunities and great people, that even if you're too early, the net result is still a pure positive for your career and startup.
The worst case scenario is a newly accepted YC startup with a little bit of traction... just enough traction that they aren't willing to change ideas/markets and not enough traction for them to actually know they have product market fit. It's the uncanny valley of product-market fit. These companies with a little bit of progress can spend months or years of their life chasing what they later realize was a mirage.
When a new YC company enters the batch with very little or no traction (and can move incredibly fast) they will longterm outperform companies accepted with small traction most of the time. Based on the hundreds of companies I have personally funded at YC, speed is the single most predictive variable of if a startup will succeed - not traction at time of accept.
Doing YC at their early state was perfect because it was the perfect environment to come up with an idea like Brex.
Moving fast is something we are working at hard at. From a technical and creative perspective.
Correct me if I am wrong. When you say moving fast, do you mean being nimble and quick at pretty much everything, with the goal to find product-market fit?
May I ask which speed exactly you are referring to?
Originally it was enough to have a few people, an idea, and a prototype. I think that was the model for the most successful YC companies currently out there (Dropbox, Airbnb, Stripe, PagerDuty). Some didn't land on their actual idea on product until after the batch (Twitch).
There are certainly companies that come in with a baked product and the start of some real users/customers who then use their time in YC to juice their numbers and raise big rounds at crazy valuations right at the close of the batch. However, I think what YC offers that is unique (and a real strength) is that they back completely unknown founders very early in their process of building a product and a company and give them the connections and advice to help build something big.
The YC series A program strikes me as more of an accelerator.
Now this is my opinion. I start a startup to be my own boss and change the world on my OWN terms. Why the hell, should I go to YC to hire a boss?
To grow faster? To learn how to grow?
BULLSHITT! I can do both of those without their help -- call me arrogant, I don't care.
Do you mind if I ask what your independent success trajectory in your own business has looked like?